1. Field of the Invention
The present invention relates to a pulsed laser, comprising a first laser medium with pulsed excitation in a resonant cavity formed between two mirrors one of which is a mirror providing phase conjugation by mixing four waves, said phase conjugation mirror comprising a secondary non linear medium associated with at least one pumping laser for sending two pumping beams incident on the secondary medium in two opposite directions and at the same frequency.
2. Description of the Prior Art
A pulsed laser may be considered a laser whose gas medium, at high pressure, at least of the order of an atmosphere, is subjected to a pulsed transverse excitation. Such a laser is called a TEA (transversely excited atmospheric) laser. The pulsed transverse excitation is provided by the electrical discharge of a capacitor generally by means of two longitudinal electrodes or plates extending in the gas medium.
The advantage of TEA lasers is to emit very brief pulses which are advantageously used in particular in telemetry. Thus, for example pulses may be obtained whose main peak has a duration of the order of 10 ns, the high pressure of the gas medium allowing pulses to be obtained with an extremely steep rising front, of a duration of the order of 20 ns.
For measuring distances, TEA lasers are perfectly suitable. The measurement of the time elapsing between the emission of a TEA laser pulse and reception of its echo from a target gives the distance of this target. On the other hand, the measurement of speeds with TEA lasers raises a problem, since pulser at frequencies which are close to that of light cannot be measured. To get over this problem, a heterodyne detection is then used, well known to anyone skilled in the art by beating, with the emission, a conventional low pressure continuous wave laser, whose emission frequency can be perfectly well stabilized.
The emission of a TEA laser is effected in several simultaneous longitudinal modes corresponding to the different frequencies of the spontaneous photons stimulating the photon avalanche at frequencies compatible with the length of the resonant cavity of the laser. A TEA laser is thus a multimode laser with a wide amplification band.
Beating between a TEA laser and a continuous wave laser is effected in the central mode of the TEA laser, the TEA laser having been previously transformed in a way known per se into a monomode pulsed laser so as not to lose the energy of the other longitudinal modes of a TEA laser. Hybrid lasers, frequency injection lasers or short cavity lasers are known as monomode TEA lasers, for example, from the article by Scott et al entitled a "Stabilization of singe mode TEA laser", and published in Optics Communications, Vol. 50, No. 5, July 1, 1984. Since the invention does not concern the transformation of a multimode TEA laser into a monomode TEA laser it is pointless here to dwell further on this technique of adaptation.
Although, as was mentioned above, it is perfectly possible to control the frequency of a continuous wave laser, it is absolutely not so for the frequency of the central mode of known TEA lasers and more generally for the frequency of pulsed lasers, because the laser gas medium is in full expansion at the moment when the laser emission appears and when the optical length of the cavity varies. Even if the frequency of the continuous wave selection laser of a frequency injection laser were somewhat varied for example, the emission frequency of the pulsed laser would always correspond to the natural frequency of the mode of the pulsed cell which was the closest to the frequency of the continuous wave selection laser.
It can thus be readily understood that beating between a continuous wave laser and a pulsed laser with an unstabilized frequency may give rise to a quite prejudicial sliding phenomenom (chirp) and which it is therefore natural to desire to eliminate.
Phase conjugate or frequency shift mirrors are also known. These are solid or liquid gas media with a non linear absorption coefficient whose function is to reflect for example, an incident wave front with partial phase shift, i.e. provided with a distortion, as a reflected wave front with partial but inverted or reversed or conjugate phase shift.
The function of these phase conjugate mirrors is perfectly well described in the prior art, more especially in U.S. Pat. No. 4,233,571.
Conjugate mirrors are already known, more especially and once again from this U.S. patent, called four wave mixing phase conjugate mirrors, comprising a non linear medium and at least one external pumping laser, generally two, for emitting two pumping wave fronts incident on the non linear medium at the same frequency and in two directions opposite each other. A third wave front incident on the non linear medium, but provided with a phase or frequency distortion, is reflected under the action of the three incident wave fronts as a fourth phase conjugate wave front.
Thus, a degenerated or distorted wave front at frequency .omega.+.delta., after for example undergoing a distortion+.delta. at the level of a laser medium, and incident on such a mirror, is reflected, as in all other phase conjugate mirrors, as a wave front at frequency .omega.-.delta.. On its return path, and after passing through the laser medium, the reflected wave front which again has undergone the same distortion+.delta. returns then to the frequency .omega.. The function of the non linear medium device associated with two pumping lasers of U.S. Pat. No. 4,233,571 is therfore to obtain a wave front correction. Lasers associated with a non linear medium only provide a pumping function. This device aims at purifying the central emission mode of a laser.
It will be noted consequently that the problem of frequency stabilization set forth above is not at all dealt with in this U.S. patent of the prior art.
The article "A theoretical and experimental investigation of the modes of optical resonants with phase conjugate mirrors" by Auyeung et al, appearing in the IEEE Journal of Quantum Electronics, vol. QE-15, No. 10, October 1979, teaches recopying into a first pulsed laser the frequency of a second pulsed pumping laser from a phase conjugate mirror. However, this article is not concerned with stabilizing the frequency of the first pulsed laser and all the more so since the frequency of the second pulsed laser is not in itself stable.
The article "Demonstration of the longitudinal modes and aberration-correction properties of a continuous wave dye laser with a phase conjugate mirror" by Lind et al, appearing in the review Optics Letters, Vol. 6, No. 11, November 1981, teaches stabilizing a continuous wave laser at the frequency of a continuous wave pumping laser of a phase conjugate mirror. However, no teaching may be drawn from this article for stabilizing a pulsed laser.
The problem of frequency stabilization of a pulsed laser remains then unsolved, and it is this problem which the applicant has solved.